Passing data from handler to middleware after serving request - api

Question

I have the following simple API in Go:
package main
import (
"context"
"fmt"
"net/http"
"github.com/gorilla/mux"
)
func middleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// Call the handler
next.ServeHTTP(w, r)
// Retrieve custom data from the request object after the request is served
customData := r.Context().Value("custom_data")
fmt.Println("Custom data:", customData)
})
}
func handler(w http.ResponseWriter, reqIn *http.Request) {
reqIn = reqIn.WithContext(context.WithValue(reqIn.Context(), "custom_data", true))
}
func main() {
r := mux.NewRouter()
// Attach the middleware to the router
r.Use(middleware)
// Attach the handler to the router
r.HandleFunc("/", handler).Methods("GET")
http.ListenAndServe(":8080", r)
}
I expected the context in the middleware to be able to access the value of "custom_data", but it is not able to, returning for that context value.
This happens even if I use Clone instead of WithContext for adding a value in the context of the request.
Looking around, specifically this post, if I instead use this as the handler:
func handler(w http.ResponseWriter, reqIn *http.Request) {
req := reqIn.WithContext(context.WithValue(reqIn.Context(), "custom_data", true))
*reqIn = *req
}
It works as expected.
But modifying the *http.Request is not the norm.
My real question that I am trying to solve is; how can I pass information from the handler to the middleware?
Adding a value to the context of the *http.Request would be able to be accessed in the middleware.

Answer 1

You can do the following:
func middleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
custom_data := make(map[string]any)
r = r.WithContext(context.WithValue(r.Context(), "custom_data", custom_data))
// Call the handler
next.ServeHTTP(w, r)
// Retrieve custom data from the request object after the request is served
v := r.Context().Value("custom_data")
fmt.Printf("Custom data(%T): %v\n", v, v)
// or use the above defined map directly
fmt.Printf("Custom data(%T): %v\n", custom_data, custom_data)
})
}
func handler(w http.ResponseWriter, r *http.Request) {
m, ok := r.Context().Value("custom_data").(map[string]any)
if ok && m != nil {
m["value"] = true
}
}